_with_resolved_session_model was overwriting the nested
input_audio_transcription.model and audio.input.transcription.model with the
realtime conversation model, silently replacing a caller's transcription model
(e.g. whisper-1) since those are a different model than the realtime deployment.
It now only resolves the top-level session model.
Also restores session.model taking precedence over the top-level model in
acreate_realtime_client_secret, matching the proxy's own
_prepare_client_secret_session ordering and avoiding a backwards-incompatible flip.
Adds routing coverage for arealtime_calls (api_base resolution) and
acreate_realtime_transcription_session (api_key resolution) so all three realtime
HTTP endpoints have router credential-resolution tests, plus regression tests for
the two fixes above.
Co-authored-by: Cursor <cursoragent@cursor.com>
